Custom Data Connector Joined to Cloud Data Source

I have a very specific issue and I wanted to ask if this is supported yet and if there are any workarounds.

I have created a custom data connector and added that to a Personal On-Premise Gateway so it can be refreshed in Power BI Service. (This works great and I'm really looking forward to when this is possible on the non-personal gateway.)

I then created a report in Power BI desktop which used both my custom connector and a SharePoint hosted Excel file as a data source (specifically choosing "Web" when getting the Excel file and just providing the full URL to the file in SharePoint.)

Publishing this report to Power BI Service online also works - I can refresh the dataset associated to the report, and it refreshes both the SharePoint file's data and the data from my custom connector.

Finally, I created a table in Power BI Desktop through the query editor with Power Query that joins the two tables together on a column like this:

Table.Join(CustomConnectorTable, "Id", SharePointTable, "Id")

After publishing this report, I get this error when trying to refresh the data:

"The import <my connector>.Contents matches no exports. Did you miss a module reference? Table:<my joined table>".

I have even tried not joining the table and just mentioning it in the Advanced Query editor like this:

let
Source = CustomConnectorTable,
Other = SharePointTable
in
Source

But even when I don't actually use both tables, the same error occurs.

When I publish the two tables separately the refresh works so the error does not occur.

I have actually worked around the issue for my particularly use case by using DAX calculated tables. Joining the tables through DAX does not cause the issue either - only in Power Query in the query editor. However, it would still be great if this could be supported in Power Query too.

As the issue has custom connector involved, it's hard to provide further support on forum side. I would suggest you create a support ticket to Microsoft and let engineers look into your real environment.